include/hw/irq.h: New function qemu_irq_is_connected()

include/hw/irq.h: New function qemu_irq_is_connected()

Mostly devices don't need to care whether one of their output
qemu_irq lines is connected, because functions like qemu_set_irq()
silently do nothing if there is nothing on the other end.  However
sometimes a device might want to implement default behaviour for the
case where the machine hasn't wired the line up to anywhere.

Provide a function qemu_irq_is_connected() that devices can use for
this purpose.  (The test is trivial but encapsulating it in a
function makes it easier to see where we're doing it in case we need
to change the implementation later.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200728103744.6909-2-peter.maydell@linaro.org